This invention relates to a metallized polypropylene film exhibiting superior tensile properties, resulting in superior formed bag barrier properties whereby gas and moisture barrier properties are maintained despite high tensions and stresses caused by packaging machinery.
This film has excellent oxygen and moisture barrier, and superior formed bag oxygen and moisture barrier properties.
A biaxially oriented polypropylene metallized film used for packaging applications often performs multiple functions. It must perform in a lamination to provide light, moisture, and oxygen barrier for gas-flush applications; it must provide a heat sealable inner layer for bag forming and sealing; and it must maintain sufficient oxygen and moisture barrier properties after bag-forming and filling in order to retain the benefits of gas-flushing salty snacks such as potato chips.
Typical metallized films used in vertical-form-fill-seal (VFFS) bag makers often demonstrate excellent barrier properties when in unlaminated or laminated form prior to bag-making. However, during the process of bag-forming, due to the wide variety of forming collars used, bag sizes, filling speeds, and machine tensions employed, the laminated structure can be stretched in the machine direction anywhere from 5-10% beyond the normal dimension of the lamination. This elongation during bag-forming can fracture or crack the metal layer of the metallized film, causing a loss of barrier properties, particularly oxygen barrier. Thus, what may have been a metallized film lamination with excellent gas barrier properties prior to bag-forming, may actually result in formed and filled bags with poor gas barrier, thus losing the benefits of the originally designed high gas barrier film.
The objective of this invention is to solve the aforesaid problems of conventional biaxially oriented polypropylene (BOPP) metallized films by providing a novel BOPP metallized film with exceptionally high tensile properties which in turn, provides greater resistance to the tensions and forces that can be exerted by typical packaging machines. This will result in a metallized high barrier packaging film with excellent formed bag barrier properties. In addition, the film can also offer excellent sealability or slip properties.
According to this invention, the above objective is achieved by a propylene polymer film comprising of at least a single layer extruded film and a vapor-deposited metal layer, with said extruded propylene polymer film exhibiting machine direction tensile properties that are in the range of about 350,000 to about 400,000 psi or higher Young""s modulus, about 110% or lower elongation at break (% strain at ultimate), and 27,000 to 30,000 psi or higher tensile strength (stress at ultimate). Such a metallized BOPP film exhibiting these superior tensile properties in the machine direction demonstrates superior resistance to the machine direction stresses and tensions exerted upon it from packaging machines. This superior machine direction resistance then translates into superior barrier properties after bag-forming.
The metallizing surface of said layer is also modified by treatment with a corona discharge or flame treatment method to enhance adhesion and wettability of the vapor-deposited metal; the surface opposite said treated metallizing surface can be treated or left untreated as desired. Additionally, other polyolefin-based resin layers may be coextruded with the primary propylene polymer film layer to enhance metallizing properties and/or, other layers coextruded opposite the metallizing surface to provide sealability or as a winding surface whose surface may also be modified with a corona discharge or flame treatment method to make it suitable for laminating or converter applied adhesives and inks.